Time-Domain Based Composite Modulation for Spectrally Efficient Optical Networks

ABSTRACT

Methods, systems, and apparatuses for time-based composite modulation of an optical carrier signal are provided. Time-based composite modulation includes determining a plurality of fixed time slots for the optical carrier signal, wherein the plurality of fixed time slots comprise a time-division-multiplexing frame. Determining a modulation format for each fixed time slot of the time-division-multiplexing frame, wherein a transport spectral efficiency of the modulation format determined for a first fixed time slot is different from a transport spectral efficiency of the modulation format determined for a second fixed time slot, and determining a number of binary bits for each fixed time slot of the time-division-multiplexing frame, wherein the number of binary bits for a fixed time slot is based on the modulation format determined for the fixed time slot.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/307,716, filed Nov. 30, 2011, now U.S. Pat. No. 8,855,495, issued onOct. 7, 2014, incorporated herein by reference.

TECHNICAL FIELD

This specification relates generally to optical carrier signalmodulation, and more particularly to methods for time-domain basedmodulation.

BACKGROUND

A major force driving the technology evolution of fiber-optic transportnetworks is the desire to lower the cost per transmitted bit. With therapid growth of capacity demands and limited available opticalbandwidth, maximizing transport spectral efficiency is becomingincreasingly important for lowering such costs. One focus of transportspectral efficiency is to improve on the capacity limitations of commonnetwork infrastructure, such as the fiber-optic cable.

In recent years, various high-order modulation formats with differentoptical reach capabilities have been proposed for increasing transportspectral efficiency. However, many of these approaches put a severelimitation on the design of optical networks, where a variety of reachdemands may be required for different wavelength channels and atdifferent points in time.

SUMMARY

A time-domain based composite modulation method using a single carrieris presented. The method is capable of achieving an arbitrary transportspectral efficiency, and therefore enables maximized spectral efficiencyfor any reach demand in an optical network. This capability can help toimprove optical signal transport economics by allowing for a moreuniform trade-off between cost per transmitted bit and reach.

In accordance with an embodiment, a method for time-based compositemodulation is provided. An optical carrier signal is received and aplurality of fixed time slots for the optical carrier signal aredetermined, wherein the plurality of fixed time slots comprise atime-division-multiplexing frame. A modulation format is determined foreach fixed time slot of the time-division-multiplexing frame, wherein atransport spectral efficiency of the modulation format determined for afirst fixed time slot is different from a transport spectral efficiencyof the modulation format determined for a second fixed time slot. Anumber of binary bits are determined for each fixed time slot of thetime-division-multiplexing frame, wherein the number of binary bits fora fixed time slot is based on the modulation format determined for thefixed time slot.

In accordance with an embodiment, the modulation format determined for afixed time slot may be one of a PDM-4QAM, PDM-8QAM, PDM-16QAM, PDM-32QAMand PDM-64QAM modulation format.

In accordance with an embodiment, a frame sync marker for thetime-division-multiplexing frame is generated, wherein the frame syncmarker indicates the modulation format determined for at least one fixedtime slot of the time-division-multiplexing frame.

In accordance with an embodiment, a packet comprising at least onetime-division-multiplexing frame is generated, and a modulation formatis determined for each fixed time slot of the packet.

These and other advantages of the present disclosure will be apparent tothose of ordinary skill in the art by reference to the followingdetailed description and the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing typically realizable PDM M-ary QAM transportspectral efficiencies;

FIG. 2 is a diagram showing the structure of atime-division-multiplexing (TDM) frame in accordance with an embodiment;

FIG. 3A is a diagram showing the structure of atime-division-multiplexing (TDM) transmitter in accordance with anembodiment;

FIG. 3B is a diagram showing the structure of atime-division-multiplexing (TDM) receiver in accordance with anembodiment;

FIG. 4 is a flowchart of a process for determining time-domain basedcomposite modulation in accordance with an embodiment;

FIG. 5 is a diagram showing a reconfigurable optical add-dropmultiplexer (ROADM) based optical network environment for implementingtime-domain based composite modulation in accordance with an embodiment;

FIG. 6 is a diagram showing the structure of time-division-multiplexing(TDM) packets in accordance with an embodiment;

FIG. 7 illustrates a packet-switched optical network for implementingtime-domain based composite modulation in accordance with an embodiment;and

FIG. 8 is a high-level block diagram of an exemplary computer that maybe used for time-domain based composite modulation.

DETAILED DESCRIPTION

FIG. 1 is a diagram showing typically realizable PDM M-ary QAM transportspectral efficiencies. One way to increase transport spectral efficiencyis to use polarization-division-multiplexed (PDM) high-order M-aryquadrature amplitude modulation (QAM). The achievable spectralefficiency for an M-ary modulation format is given by 2 log₂M using PDM,where M is typically required to be equal to 2^(k) (k is an integernumber) for practical implementation consideration. As shown, using asingle PDM M-ary QAM typically limits achievable transport spectralefficiencies (sometimes denoted as “SE” or “SEs” herein) to discreteeven numbers (e.g., SE=4 for k=2, SE=6 for k=3, and SE=9 for k=4, etc.),implying that the optimal spectral efficiency cannot be realized formany reach demands by using a single PDM M-ary QAM. This limitationoften translates into poorer transport economics because the reachdemands in a fully dynamic optical network can be quite different fordifferent wavelength channels. Even for the same wavelength channel, thereach demand may also vary over time.

To realize an “arbitrary” (i.e., maximized) spectral efficiency foroptical transport, a time-domain based composite modulation method isprovided. The method applies different modulation formats with differentSEs to the same optical carrier signal, but at different time slots. Dueto the use of a single carrier signal, the method enables a lower signalpeak-to-average power ratio (e.g., as compared to typical orthogonalfrequency-division multiplexing (OFDM) based methods) and therefore canimprove tolerance of fiber non-linearities. Furthermore, the method ismore tolerant toward laser phase noise because the symbol period is muchshorter than frequency-domain based composite modulation methods.

FIG. 2 is a diagram showing the structure of time-division-multiplexing(TDM) frames in accordance with an embodiment. Frames 200 and 202include a plurality of serial high-speed binary bits 204, which aredivided among the fixed time slots (i.e., tributaries) of TDM frames 200and 202. In one embodiment, TDM frames 200 and 202 may each includethree fixed time slots. The binary bits 204 may be apportioned among thefixed time slots based on the modulation format determined for eachfixed time slot. While one skilled in the art will appreciate that themethods described herein are suitable for any number of frame andmodulation formats, for ease of understanding, TDM frames 200 and 202are constructed using two standard modulation formats, PDM-4QAM andPDM-8QAM. For example, the first 6 binary bits 206 can be assigned tothe first fixed time slot 208 of frame 200, which is mapped into asingle PDM-8QAM symbol. The second 4 binary bits 210 can be assigned tothe second fixed time slot 212, which is mapped into a single PDM-4QAMsymbol. Likewise, the third 4 binary bits 214 can be assigned to thethird fixed time slot 216, which is mapped into another PDM-4QAM symbol.After all of the binary bits are mapped to a corresponding PDM-M-QAMsymbol, each TDM frame 200 and 202 of the compositely modulated signalconsists of one PDM-M-8QAM symbol and two PDM-4QAM symbols, achieving anet transport SE given by:

(1/3)·SE(PDM-8QAM)+(2/3)·SE(PDM-4QAM)=14/3;

where SE(x) denotes the theoretical SE of x in terms of the number ofbits per symbol.

For such a time-domain based composite modulation method, if a TDM frameconsists of L time slots (time slot length=a symbol period), and ofwhich N time slots are assigned with modulation format 1 and the otherL-N time slots are assigned with modulation format 2, then the net SE isgiven by:

(N/L)·SE(format 1)+(1−N/L)·SE(format 2).

As such, an arbitrary SE can be achieved by using two modulation formatswith appropriate values of L and N, when the demanded SE is locatedbetween the SE of modulation format 1 and the SE of modulation format 2.

In one embodiment, the Euclidean distances for constellation points ofthe individual (or component) modulation formats used in the proposedcomposite modulation method (e.g. d₁ and d₂ in FIG. 2) can be designedto be either identical or different from each other. In the case of alinear transmission channel, an equal Euclidean distance might givebetter performance than unequal Euclidean distance. For a nonlineartransmission channel, such as a fiber-optic transmission channel,however, an unequal Euclidean distance might yield better nonlineartolerance. An unequal Euclidean distance design may also help improvephase noise tolerance. In one embodiment, the Euclidean distances of thecomponent modulation formats may be optimized to improve the overallnonlinear tolerance and/or phase noise tolerance. Moreover, thedistribution of the component modulation formats across a TDM frame canalso be optimized to improve the nonlinear tolerance as well as forwarderror correction (FEC) coding gain. Generally, a better nonlinearperformance may be achieved by interleaving a higher-SE componentmodulation format with a lower-SE component modulation format (e.g.,ababab or abbabb, where a denotes component modulation 1 and b denotescomponent modulation 2). Such interleaving may also improve the laserphase tolerance for optical communication systems using coherentdetection. Furthermore, a joint design and optimization of FEC codingand time-domain composite modulation may enhance the overall performanceor simplify the FEC decoding (thus reduce latency) by exploiting theknowledge of different component modulation formats having differentprobabilities of error.

In another embodiment, a frame sync marker for thetime-division-multiplexing frame is generated indicating the modulationformat for at least one fixed time slot of thetime-division-multiplexing frame. For example, the frame sync marker mayindicate the modulation formats within a TDM frame to a decoder todecrease decoding latency.

FIG. 3A is a diagram showing the structure of atime-division-multiplexing (TDM) transmitter and FIG. 3B is a diagramshowing the structure of TDM receiver in accordance with an embodiment.In FIG. 3A, at transmitter 300, the time-domain based compositemodulation method can be implemented using digital methods. For example,a CMOS-based digital signal processing (DSP) module 302 can beconfigured to generate a compositely-modulated electrical signal.Digital-to-analog converter 304 can be configured to convert the digitalsignal into an analog signal, which then can be used to drive opticalmodulator 306 to generate (in conjunction with laser 308) acompositely-modulated optical signal. The compositely-modulated opticalsignal may then be received at receiver 310 shown in FIG. 3B. Receiver310 can be configured to detect and demodulate the compositely-modulatedoptical signal using, for example, means for digital coherent detectionsuch as receiving CMOS-based DSP module 312.

FIG. 4 is a flowchart of a process for determining time-domain basedcomposite modulation in accordance with an embodiment. In oneembodiment, processing module 302 may include circuits configured toperform operations for generating a compositely-modulated digitalsignal. For example, At 400, processing module 302 determines aplurality of fixed time slots for an optical carrier signal, wherein thefixed time slots 208, 212, and 216 comprise a TDM frame 200, such as TDMframe 200 in FIG. 2 above. At 402, processing module 302 determines amodulation format for each fixed time slot of thetime-division-multiplexing frame. For example, the modulation formatdetermined for a fixed time slot may be one of a PDM-4QAM, PDM-8QAM,PDM-16QAM, PDM-32QAM and PDM-64QAM modulation format, or another knownmodulation format. Further, the modulation format determined for a firstfixed time slot can be different from the modulation format determined asecond fixed time slot. As shown in FIG. 2, fixed time slot 208 is aPDM-8QAM time slot, while fixed time slot 212 is a PDM-4QAM time slot.At 404, processing module 302 determines a number of binary bits foreach fixed time slot of the time-division-multiplexing frame.Specifically, the number of binary bits for a fixed time slot is basedon the modulation format determined for the fixed time slot. Forexample, the PDM-4QAM time slots in FIG. 2 include 4 binary bits each,while the PDM-8QAM time slots include 6 binary bits.

As shown in FIG. 3A above, digital-to-analog converter 304 includes adigital signal input coupled to an output of processing module 302, andprovides an analog signal output. As such, at 406, digital-to-analogconverter 304 converts the TDM frame, a compositely-modulated digitalsignal generated by processing module 302, into a compositely-modulatedanalog signal which is received by optical modulator 306. For example,optical modulator 306 includes an analog signal input coupled to theanalog signal output of digital-to-analog converter 304 for receivingcompositely-modulated analog signals, and provides an optical signaloutput. At 408, optical modulator 306 generates a compositely-modulatedoptical signal, in conjunction with laser 308 based on thecompositely-modulated analog signal converted by digital-to-analogconverter 304.

FIG. 5 is a diagram showing a reconfigurable optical add-dropmultiplexer (ROADM) based optical network environment for implementingtime-domain based composite modulation in accordance with an embodiment.For example, network 500 includes transmitter node 502 and receivingnodes 504 and 506, which are located at various transmission distancesfrom transmitter node 502. Network 500 may also include one or moreROADMs 508 for individual or multiple wavelengths carrying data channelstransmitted from transmitter node 502 (or other nodes) to be added ordropped from a transport fiber. In such a network, it would beadvantageous to optimize spectral efficiency based on transmission reach(and other line system conditions) with the time-domain based compositemodulation method of FIG. 2. For example, a method for optimizingspectral efficiency can be advantageous when the transmission reachbetween transmitter node 502 and a first receiver node 504 is longer(with lower transport spectral efficiency) than the transmission reachbetween transmitter node 502 and a second receiver node 506. In suchcases, the method can be employed by transmitter 502 to realize maximumSE for each wavelength channel 510 and 512 for the reach demands oftransmitting to receiver nodes 504 and 506, respectively.

FIG. 6 is a diagram showing the structure of time-division-multiplexing(TDM) packets in accordance with an embodiment. In one embodiment,binary bits 600 can be grouped into packets based on transmission reachor other demands. For example, packets 602 and 604 may have differentdestinations and therefore the transmission reach demands. To maximizethe transport SE for each individual packet, different modulationformats may be applied to different packets using the time-domain basedcomposite modulation method of FIG. 2. Moreover, within each packet,more than one modulation format may be applied using the method of FIG.2 as shown at 606.

FIG. 7 illustrates a packet-switched optical network for implementingtime-domain based composite modulation in accordance with an embodiment.Packet-switched optical communication network 700 includes transmitternode 702 and receiving nodes 704 and 706, which are located at varioustransmission distances from transmitter node 702. Network 700 may alsoinclude one or more optical packet switches 708 for adding or droppingpackets from a transport fiber. In network 700, the transport SE of anyindividual packet can be optimized using the time-domain based compositemodulation method of FIG. 2 based on transmission reach or otherdemands. For example, the transport SE of Packet 1 710 can be optimizedusing the time-domain based composite modulation method of FIG. 2 basedon the higher SE transmission reach between transmitter node 702 andreceiver node 706. Likewise, the transport SE of Packet 2 712 can beoptimized using the method of FIG. 2 based on the lower SE transmissionreach between transmitter node 702 and receiver node 704.

As such, a time-domain based composite modulation method is provided forvarious high-spectral-efficiency and high-speed optical networks. Oneskilled in the art will appreciate that the network examples of FIGS. 5and 7 are illustrative and that the method of FIG. 2 may be employed ina variety of other networks. Moreover, it will be appreciated that themethod also may be employed in a combination of different types ofnetworks.

The method allows for arbitrary transport spectral efficiency by using asingle carrier with multiple common M-ary (M=2^(k), k is an integernumber) modulation formats that each have a different SE because theyhave a different number of bits per symbol. Moreover, a transmission canbe at a fixed modulation rate (baud) to minimize implementation costs.The method maximizes the transport SE for any transmission reach (e.g.,maximum transmission distance without regeneration), thus reducing thecost per transmitted bit.

Unlike frequency-domain based composite modulation methods, wheredifferent modulation formats are applied to different subcarriers, themethod applies different modulation formats to a single carrier, but indifferent time slots (where a one time slot is equal to a symbolperiod). Multiple adjacent time slots can be grouped to form atime-division-multiplexing (TDM) frame, and different modulation formatscan be applied to different time slots within each TDM frame. The samemodulation format can be applied to more than one time slot within eachTDM frame when, for example, the used number of modulation formats issmaller than the number of time slots per TDM frame. Alternatively,different modulation formats can be applied to different time slotswithin each optical packet. Moreover, the used modulation formats and/ortheir distribution within each packet can differ from packet to packet,as different packets may have different reach requirements. Also, afirst modulation format can be used for the packet header (probablylower SE but less error prone) and a second modulation format can beused for the packet payload (greater SE, more error prone). As comparedto frequency-domain based composite modulation methods, the embodimentscan enable a lower signal peak-to-average power ratio and therefore canimprove fiber nonlinear tolerance. The method is also more toleranttoward laser phase noise due to the shorter symbol period.

In various embodiments, the method steps described herein, including themethod steps described in FIG. 4, may be performed in an order differentfrom the particular order described or shown. In other embodiments,other steps may be provided, or steps may be eliminated, from thedescribed methods. In still other embodiments, the steps may be brokendown into sub-steps which may, for example, be performed in parallel.

Systems, apparatus, and methods described herein may be implementedusing digital circuitry, or using one or more computers using well-knowncomputer processors, memory units, storage devices, computer software,and other components. Typically, a computer includes a processor forexecuting instructions and one or more memories for storing instructionsand data. A computer may also include, or be coupled to, one or moremass storage devices, such as one or more magnetic disks, internal harddisks and removable disks, magneto-optical disks, optical disks, etc.

Systems, apparatus, and methods described herein may be implementedusing computers operating in a client-server relationship. Typically, insuch a system, the client computers are located remotely from the servercomputer and interact via a network. The client-server relationship maybe defined and controlled by computer programs running on the respectiveclient and server computers.

Systems, apparatus, and methods described herein may be used within anetwork-based cloud computing system. In such a network-based cloudcomputing system, a server or another processor that is connected to anetwork communicates with one or more client computers via a network. Aclient computer may communicate with the server via a network browserapplication residing and operating on the client computer, for example.A client computer may store data on the server and access the data viathe network. A client computer may transmit requests for data, orrequests for online services, to the server via the network. The servermay perform requested services and provide data to the clientcomputer(s). The server may also transmit data adapted to cause a clientcomputer to perform a specified function, e.g., to perform acalculation, to display specified data on a screen, etc. For example,the server may transmit a request adapted to cause a client computer toperform one or more of the method steps described herein, including oneor more of the steps of FIG. 4. Certain steps of the methods describedherein, including one or more of the steps of FIG. 4, may be performedby a server or by another processor in a network-based cloud-computingsystem. Certain steps of the methods described herein, including one ormore of the steps of FIG. 4, may be performed by a client computer in anetwork-based cloud computing system. The steps of the methods describedherein, including one or more of the steps of FIG. 4, may be performedby a server and/or by a client computer in a network-based cloudcomputing system, in any combination.

Systems, apparatus, and methods described herein may be implementedusing a computer program product tangibly embodied in an informationcarrier, e.g., in a non-transitory machine-readable storage device, forexecution by a programmable processor; and the method steps describedherein, including one or more of the steps of FIG. 4, may be implementedusing one or more computer programs that are executable by such aprocessor. A computer program is a set of computer program instructionsthat can be used, directly or indirectly, in a computer to perform acertain activity or bring about a certain result. A computer program canbe written in any form of programming language, including compiled orinterpreted languages, and it can be deployed in any form, including asa stand-alone program or as a module, component, subroutine, or otherunit suitable for use in a computing environment.

A high-level block diagram of an exemplary computer that may be used toimplement systems, apparatus and methods described herein is illustratedin FIG. 8. Computer 800 includes a processor 801 operatively coupled toa data storage device 802 and a memory 803. Processor 801 controls theoverall operation of computer 800 by executing computer programinstructions that define such operations. The computer programinstructions may be stored in data storage device 802, or other computerreadable medium, and loaded into memory 803 when execution of thecomputer program instructions is desired. Thus, the method steps of FIG.4 can be defined by the computer program instructions stored in memory803 and/or data storage device 802 and controlled by the processor 801executing the computer program instructions. For example, the computerprogram instructions can be implemented as computer executable codeprogrammed by one skilled in the art to perform an algorithm defined bythe method steps of FIG. 4. Accordingly, by executing the computerprogram instructions, the processor 801 executes an algorithm defined bythe method steps of FIG. 4. Computer 800 also includes one or morenetwork interfaces 804 for communicating with other devices via anetwork. Computer 800 also includes one or more input/output devices 805that enable user interaction with computer 800 (e.g., display, keyboard,mouse, speakers, buttons, etc.).

Processor 801 may include both general and special purposemicroprocessors, and may be the sole processor or one of multipleprocessors of computer 800. Processor 801 may include one or morecentral processing units (CPUs), for example. Processor 801, datastorage device 802, and/or memory 803 may include, be supplemented by,or incorporated in, one or more application-specific integrated circuits(ASICs) and/or one or more field programmable gate arrays (FPGAs).

Data storage device 802 and memory 803 each include a tangiblenon-transitory computer readable storage medium. Data storage device802, and memory 803, may each include high-speed random access memory,such as dynamic random access memory (DRAM), static random access memory(SRAM), double data rate synchronous dynamic random access memory (DDRRAM), or other random access solid state memory devices, and may includenon-volatile memory, such as one or more magnetic disk storage devicessuch as internal hard disks and removable disks, magneto-optical diskstorage devices, optical disk storage devices, flash memory devices,semiconductor memory devices, such as erasable programmable read-onlymemory (EPROM), electrically erasable programmable read-only memory(EEPROM), compact disc read-only memory (CD-ROM), digital versatile discread-only memory (DVD-ROM) disks, or other non-volatile solid statestorage devices.

Input/output devices 805 may include peripherals, such as a printer,scanner, display screen, etc. For example, input/output devices 805 mayinclude a display device such as a cathode ray tube (CRT) or liquidcrystal display (LCD) monitor for displaying information to the user, akeyboard, and a pointing device such as a mouse or a trackball by whichthe user can provide input to computer 800.

Any or all of the systems and apparatus discussed herein, includingtransmitter 300, and components thereof, including processing module302, digital-to-analog converter 304, and optical modulator 306 may beimplemented using a computer such as computer 800.

One skilled in the art will recognize that an implementation of anactual computer or computer system may have other structures and maycontain other components as well, and that FIG. 8 is a high levelrepresentation of some of the components of such a computer forillustrative purposes.

The foregoing Detailed Description is to be understood as being in everyrespect illustrative and exemplary, but not restrictive, and the scopeof the invention disclosed herein is not to be determined from theDetailed Description, but rather from the claims as interpretedaccording to the full breadth permitted by the patent laws. It is to beunderstood that the embodiments shown and described herein are onlyillustrative of the principles of the present disclosure and thatvarious modifications may be implemented by those skilled in the artwithout departing from the scope and spirit of this disclosure. Thoseskilled in the art could implement various other feature combinationswithout departing from the scope and spirit of this disclosure.

1-14. (canceled)
 15. A method comprising: determining a plurality offixed time slots defining a time-division-multiplexing frame for anoptical carrier signal; determining a modulation format for each fixedtime slot of the time-division-multiplexing frame; determining a numberof binary bits for each fixed time slot of thetime-division-multiplexing frame; and generating a frame sync marker forthe time-division-multiplexing frame based on a determined modulationformat.
 16. The method of claim 15, wherein the modulation formatdetermined for a fixed time slot is one of a PDM-4QAM, PDM-8QAM,PDM-16QAM, PDM-32QAM and PDM-64QAM modulation format.
 17. The method ofclaim 15, wherein one or more time-division-multiplexing frames define apacket.
 18. The method of claim 15, wherein a transport spectralefficiency of the modulation format determined for a first fixed timeslot is different from a transport spectral efficiency of the modulationformat determined for a second fixed time slot.
 19. The method of claim15, wherein the number of binary bits for a fixed time slot is based onthe modulation format determined for the fixed time slot.
 20. The methodof claim 15, wherein the modulation format is determined for a fixedtime slot of the time-division-multiplexing frame.
 21. An apparatuscomprising: a processing module; a computer-readable medium storingcomputer program instructions for generating a compositely-modulateddigital signal, which, when executed on the processing module, cause theprocessing module to perform operations comprising: determining aplurality of fixed time slots defining a time-division-multiplexingframe for an optical carrier signal; determining a modulation format foreach fixed time slot of the time-division-multiplexing frame;determining a number of binary bits for each fixed time slot of thetime-division-multiplexing frame based on the modulation formatdetermined for the fixed time slot to generate a compositely modulateddigital signal, wherein the modulation format determined for a firstfixed time slot of the compositely modulated digital signal is differentfrom the modulation format determined for a second fixed time slot; adigital-to-analog converter, having a digital signal input coupled to anoutput of the processing module and providing an analog signal output,for converting the compositely-modulated digital signal into acompositely-modulated analog signal; and an optical modulator, having ananalog signal input coupled to the analog signal output of thedigital-to-analog converter and providing an optical signal output, forgenerating a compositely-modulated optical signal based on thecompositely-modulated analog signal.
 22. The apparatus of claim 21,wherein the modulation format determined for a fixed time slot is one ofa PDM-4QAM, PDM-8QAM, PDM-16QAM, PDM-32QAM and PDM-64QAM modulationformat.
 23. The apparatus of claim 21, further comprising computerprogram instructions causing the processing module to perform anoperation for generating a frame sync marker for thetime-division-multiplexing frame, wherein the frame sync markerindicates the modulation format determined for a fixed time slot of thetime-division-multiplexing frame.
 24. The apparatus of claim 21, whereinone or more time-division-multiplexing frames define a packet.
 25. Theapparatus of claim 21, wherein the processing module is implemented by aCMOS-based digital signal processing chip.
 26. The apparatus of claim21, wherein the processing module and the digital-to-analog converterare integrated on a CMOS-based chip.
 27. A non-transitorycomputer-readable medium storing computer program instructions forgenerating a compositely-modulated digital signal, which, when executedon a processor, cause the processor to perform operations comprising:receiving an optical carrier signal; determining a plurality of fixedtime slots defining a time-division-multiplexing frame for an opticalcarrier signal; determining a modulation format for each fixed time slotof the time-division-multiplexing frame; determining a number of binarybits for each fixed time slot of the time-division-multiplexing frame;and generating a frame sync marker for the time-division-multiplexingframe based on a determined modulation format.
 28. The non-transitorycomputer-readable medium of claim 27 wherein the modulation formatdetermined for a fixed time slot is one of a PDM-4QAM, PDM-8QAM,PDM-16QAM, PDM-32QAM and PDM-64QAM modulation format.
 29. Thenon-transitory computer-readable medium of claim 27, wherein one or moretime-division-multiplexing frames define a packet.
 30. Thenon-transitory computer-readable medium of claim 27, wherein a transportspectral efficiency of the modulation format determined for a firstfixed time slot is different from a transport spectral efficiency of themodulation format determined for a second fixed time slot.
 31. Thenon-transitory computer-readable medium of claim 27, wherein the numberof binary bits for a fixed time slot is based on the modulation formatdetermined for the fixed time slot.
 32. The non-transitorycomputer-readable medium of claim 27, wherein the modulation format isdetermined for a fixed time slot of the time-division-multiplexingframe.